This invention relates generally to apparatus and methods for measuring power in a circuit, and more particularly, to methods and apparatus for measuring power in a communications system.
A radio frequency (RF) communications system of the cellular type, including a wireless, cordless, or digital PCS communications system, is typically divided into substantially distinct geographical areas or cells, with a base station assigned to each cell, and with the base station configured to handle over a wireless interface communications to and from mobile units which happen to be within the cell. The system typically provides two-way communication between any two mobile units within the system. In addition, the system is typically interfaced to the Public Switched Telephone Network (PSTN), that is, the normal wired telephone network, and allows two-way communication between telephones wired to the PSTN and mobile units within the system.
In such a system, there is a need to regulate the power output of a mobile unit in order to conserve power, and to avoid saturation of the base station. Consider, for example, a cellular system in which two mobile units such as cell phones are within a particular cell, with one of the mobile units situated close to the base station, and the other of the mobile units being situated far from the base station. If the two mobile units transmitted to the base station at the same power, there is a danger that the mobile unit close to the base station would drown out the mobile unit far from the base station, that is, saturate the base station, since the received power from that unit would be large relative to that of the other. Consequently, the base station in such a system typically monitors the signal strength of the mobile units within the cell, and issues commands to these mobile units to adjust their signal strength such that the received signals are at a substantially uniform level. The effect of this is to lower the signal strength of units close to the base station in relation to units far from the base station. In order to ensure that the mobile units are transmitting at the level commanded by the base station, a power measurement circuit is required to measure the power being transmitted by the mobile unit for comparison with the level commanded by the base station.
Another factor is conservation of power at the mobile unit. Typically, such units are powered by power sources such as batteries and the like which have limited lifetimes. If a mobile unit were to transmit at a power level exceeding that required to achieve communication with the base station, the battery life of the mobile unit would be prematurely terminated. In order to conserve battery lifetime, a power measurement circuit is again required to measure the power being transmitted by the mobile unit for comparison with the level needed to communicate with the base station.
Similar needs are present in a satellite communications system, in which a satellite is configured to handle communications to and from a plurality of mobile units over a wireless interface. As in the cellular system, the satellite communications system typically allows two-way communication between any two mobile units within the system. Typically, each satellite in the system communicates to a base station through a wireless interface.
In such a system, if a mobile unit transmitted to a satellite at a power too high in relation to the distance between the mobile unit and the satellite, the satellite might assume that the mobile unit was situated a distance from the satellite greater than the actual distance, and transmit to the mobile unit at too high a level in relation to the actual distance. The capacity of the satellite to handle communications with other base stations might thus be unnecessarily diminished. A power measurement circuit is thus needed to monitor the power of the signal transmitted by the mobile unit to ensure that it is at the appropriate level.
In mobile units such as mobile phones, in which a signal is transmitted to a base station or satellite through a forward or transmit link, a conventional power measurement circuit is based on a Schottky diode which rectifies the transmitted signal. The circuit processes the rectified signal and outputs a voltage related to the power of the signal.
A problem arises because the Schottky diode has undesirable properties, including a power to voltage relationship which is non-linear and also highly frequency dependent. These properties make it difficult to characterize the relationship between power and voltage in the conventional power measurement circuit. Under such conditions, in order to obtain accurate power measurements, it is generally necessary to calibrate each mobile unit over an extensive range of operating variables and also to have that calibration data stored with the mobile unit at all times.
However, because each mobile unit typically exhibits different and unique characteristics, such data must typically be determined for each individual unit, a time-consuming and expensive task. Moreover, the memory required to store such data can be expensive and unduly increase the cost of the mobile unit.
FIG. 1 illustrates a plot of average input power vs. output voltage over various frequencies in an exemplary embodiment of a conventional power measurement circuit. The curve 202 is a plot of power vs. voltage at a frequency f1; the curve 204, a plot of power vs. voltage at a frequency f2; and the curve 205, a plot of power vs. voltage at a frequency f3. As can be seen, for each curve, a non-linear relationship between power and voltage exists. In addition, the output voltage for a given input power is highly dependent on frequency. More specifically, for an input power P1, the output voltage for frequency f1 is V1; that for frequency f2 is V2; and that for frequency f3 is V3.
For a mobile unit exhibiting a relationship between power and voltage as represented by the curves of FIG. 1, it is generally necessary to determine and then store with the mobile unit the data representing each of these curves. This data would then be used to determine the power being transmitted based on the voltage at the output of the power measurement circuit. The cost of performing this calibration procedure, and also the cost of maintaining this data with the mobile unit, can be unduly expensive for a mobile unit, which tends to be a low-cost and high-volume device.
The problem is particularly acute with dual mode phones, that is phones which operate in two distinct frequency bands, such as the 800 MHz cellular band and the 1900 MHz digital PCS band. The typical dual mode phone incorporates two power measuring circuits, one for each frequency band. This is due, in part, to the difficulty of building a single power measuring circuit that will operate in two widely separated frequency bands. The use of two power measuring circuits increases the cost, weight and complexity of the dual mode phone.
Given the current state of the art, there is a need for apparatus and methods for measuring power in which the relationship between average input power and output voltage is a substantial linear relationship over the operating region of interest.
There is also a need for apparatus and methods for measuring power in which the relationship between average input power and output voltage is substantially invariant to frequency over the frequencies or frequency bands of interest.
There is also a need for apparatus and methods for measuring power in which a single power measurement circuit is capable of operating in two or more frequency bands.
There is also a need for apparatus and methods for measuring power that reduce the work and memory required to calibrate the typical communications device.
There is also a need for apparatus and methods for measuring power that overcomes the disadvantages of the prior art.
The objects of the subject invention include fulfillment of any the foregoing needs, singly or in combination. Additional objects and advantages will be set forth in the description which follows, or will be apparent to those of ordinary skill in the art who practice the invention.
To achieve the foregoing objects, and in accordance with the purpose of the invention as broadly described herein, there is provided a power measuring circuit comprising a multiplier coupled to a signal of interest. In one implementation, in which the multiplier is a mixer, the signal is coupled to the RF and local oscillator (LO) inputs of the mixer, and the mixer thus multiplies the signal by itself. A filter is provided for filtering the output of the multiplier. In one implementation, the output of the filter is a signal substantially representative of or relating to the average power of the signal of interest. In another implementation example, the filter is a low-pass filter for low pass filtering the output of the mixer. In a third implementation, the output of the low pass filter is a signal substantially representative of or relating to the DC component of the signal output from the multiplier. Advantageously, the relationship between average input power and output voltage in the circuit is substantially linear over the operating region of interest.
In one implementation in which the multiplier is a mixer, the mixer is operated such that the conversion loss exhibited by the mixer is substantially constant over the operating region of interest relative to a selected one of the LO and RF inputs of the mixer.
In another embodiment, an amplifier is provided to amplify the signal provided to a selected one of the first and second inputs of the multiplier. In one implementation example of this embodiment in which the multiplier is a mixer, the signal provided to the LO input of the mixer is amplified to ensure the mixer is operating in a mode in which the conversion loss exhibited by the mixer is substantially constant with respect to the level of the signal at the LO input of the mixer.
In a further embodiment of the invention, an attenuator is provided to attenuate the signal provided to a selected one of the first and second inputs of the multiplier. In one implementation example of this embodiment in which the multiplier is a mixer, the RF input of the mixer is attenuated to ensure that the mixer is operating in a mode of operation in which the conversion loss exhibited by the mixer is substantially constant with respect to the level of the signal at the RF input of the mixer.
By employing such a circuit in a communications device, the task of calibrating the device is greatly simplified. In a single mode phone, that is, a phone operating at a single frequency or frequency band, only the slope, K, characterizing the relationship between voltage and power at the frequency of operation need be stored with the device. In a dual mode phone, that is, a phone capable of operating at two distinct frequencies, only the slopes, K1 and K2, respectively characterizing the relationship between power and frequency at the two frequencies of interest need be stored with the device.
In yet another embodiment of the invention, the multiplier is such that the relationship between average input power and output voltage is substantially invariant to frequency over the desired region of operation.
By employing a power measurement circuit in accordance with this embodiment in a dual mode phone, the task of calibrating the device is even more simplified. More specifically, in such a device, only a single slope, K, characterizing the relationship between power and voltage over the desired region of operation need be stored with the device because such characterizes the relationship for both frequencies of operation.
There is also provided a related method for measuring the power of a signal of interest comprising the steps of obtaining the signal of interest; multiplying the signal by itself; and deriving from the squared signal a signal representative of the power of the signal of interest.
In a second embodiment of the method, the signal of interest is obtained; first and second signals are derived from the signal of interest; a parameter of a selected one of the first and second signals is adjusted; the adjusted signal and the other of the first and second signals are multiplied by one another to obtain a multiplied signal; and a signal representative of the power of the signal of interest is derived from the multiplied signal. In one implementation, the amplitude of the selected signal is amplified prior to the multiplication step. In another implementation, the amplitude of the selected signal is attenuated prior to the multiplication step. In one implementation example in which the multiplication step is performed by a mixer, the altering step is performed such that the conversion loss of the mixer is substantially constant relative to the amplified or attenuated input over the desired region of operation.